error[E0510]: cannot assign `*b` in match guard
  --> $DIR/fake_borrows.rs:9:16
   |
LL |     match b {
   |           - value is immutable in match guard
LL |         deref!(true) => {}
LL |         _ if { *b = true; false } => {}
   |                ^^^^^^^^^ cannot assign

error: aborting due to 1 previous error

For more information about this error, try `rustc --explain E0510`.